1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a facsimile and a printer, and more particularly to an image forming apparatus having plural image bearers and using a two-component developer including a toner and a carrier, and an image forming method using the image forming apparatus.
2. Discussion of the Background
Image forming apparatuses such as copiers, facsimiles and printers equipped with plural image bearers such as photoreceptors for the purpose of forming color images are known. Such image forming apparatuses use a direct transfer method of directly multi-transferring toner images formed by the plural image bearers onto a sheet such as a paper and fixing the toner images thereon upon application of heat, etc.; and an intermediate transfer method of multi-transferring toner images onto an intermediate transferer, transferring them onto a sheet and fixing them thereon. The intermediate transfer method is, in other words, an indirect transfer method.
For example, a full-color image forming apparatus using an intermediate transfer method is equipped with a first transferer transferring a toner image on an image bearer onto an intermediate transferer, a second transferer transferring the toner image on the intermediate transferer onto a sheet, a first cleaner cleaning a toner remaining on the image bearer after the toner image is transferred onto the intermediate transferer, and a second cleaner cleaning a toner remaining on the intermediate transferer after the toner image is transferred onto the sheet. These cleaners are typically equipped with a cleaning blade scraping the toner remaining on the image bearer and the intermediate transferer.
A two-component developer including a toner and a carrier is well known as a developer forming images. In an image forming apparatus using the two-component developer, a toner in the developer is borne by the image bearer to form a toner image, the toner image is transferred onto a sheet such as a paper, and the toner image fixed thereon upon receipt of a heat energy, etc.
A toner used for developing an electrostatic latent image is typically a colored particle formed of a binder resin including a colorant, a charge controlling agent and other additives, and is broadly classified into a pulverized toner and a chemical toner.
A toner having a smaller particle diameter produces images having higher definition and quality. The pulverized toner prepared by conventional kneading and pulverizing methods having an amorphous shape produces images having poor granularity and sharpness. In addition, having poor fluidity as a powder due to the shape, the pulverized toner needs a large amount of a fluidizer and has a low filling rate in a toner bottle, which prevents the apparatus from being downsized. Further, the pulverized toner has a minimum particle size larger than that of the chemical toner.
Further, in a full-color image forming apparatus having a complicated transfer process, i.e., a toner image is transferred from an image bearer to an intermediate transferer and to a sheet therefrom, the pulverized toner having poor transferability due to the amorphous shape causes images having blank spots and a large consumption thereof to cover the poor transferability.
In order to solve the problems of the amorphous toner, various methods of preparing spheric chemical toners are developed.
The method of preparing chemical toners include suspension polymerization methods and emulsion polymerization condensation methods. Japanese published unexamined application No. 7-152202 discloses a polymer solution suspension method. This is a method of dispersing or dissolving toner materials in a volatile solvent such as an organic solvent having a low boiling point to prepare a dispersion or a solution, emulsifying and dripping the dispersion or solution in an aqueous medium with the presence of a dispersant, and removing the volatile solvent.
Japanese published unexamined application No. 11-149179 discloses a method of using a low-molecular-weight resin in the polymer solution suspension method to decrease the viscosity of the dispersion. Therefore, the dispersibility and emulsification of the toner materials improve, and further the fixability of the resultant toner improves because the toner materials were subjected to a polymerization reaction in a particle.
Differently from the suspension polymerization methods and emulsion polymerization condensation methods, these methods can use general-purpose resins. Particularly, a polyester resin having good low-temperature fixability and effectively used for full-color processes needing transparency and image smoothness after fixed can be used. These methods are particularly called ester elongation polymerization methods.
However, a spherical toner having a small particle diameter has very poor cleanability. In addition, having good transferability, the spherical toners having smaller particle diameters adhere more to a photoreceptor, resulting in deterioration of the transferability.
Japanese published unexamined application No. 3-100661 discloses adding a medium-size inorganic particulate material having an average particle diameter of from 20 to 40 nm to an external additive in order to improve cleanability and transferability. Japanese Patents Nos. 3328013 and 3056122, and Japanese published unexamined application No. 9-319134 disclose using a large-size inorganic particulate material having an average particle diameter not less than 100 nm as an external additive to assure cleanability and to prevent other small-size additives from being buried by a stress in an image developer.
However, these initially have good cleanability and transferability, but the external additives are buried as time passes.
Recent demands for saving resources and electrical power require reduction of energy for fixing a toner, and therefore a toner being a resin powder is having a lower softening point. A toner having a lower softening point preferably includes comparatively a soft resin such as a polyester resin as a binder resin forming the toner.
It is difficult to maintain good cleanability and transferability even by the arts disclosed in Japanese published unexamined applications Nos. 3-100661 and 9-319134, and in Japanese Patents Nos. 3328013 and 3056122, because the polyester resins has a mechanical strength lower than those of other resins. Therefore, an image forming apparatus producing high-resolution and high-quality images for long periods even with a toner including a polyester resin having good low-temperature fixability and in which an external additive is likely to be buried.
Meanwhile, a toner image is required to faithfully and stably transferred from an image bearer onto an intermediate transferer, and from the intermediate transferer onto a sheet.
Therefore, a full-color image forming apparatus using the intermediate transferer occasionally has a pressurizer pressing the intermediate transferer to the image bearer. The pressurizer pressurizes between the image bearer and the intermediate transferer to increase transfer efficiency and prevent defective transfers such as hollow images. The pressurizer also prevents the intermediate transferer from waving and evenly contacts the intermediate transferer to the surface of the image bearer to prevent uneven transferer.
However, when a pressure is applied between the image bearer and the intermediate transferer, a stress is concentrated on a part of a toner image thereon, resulting in occasional defective transfers, i.e., the centers of toner images, particularly line images and letter images on the intermediate transferer drop out.
In order to prevent such defective transfers, Japanese published unexamined applications Nos. 2003-098770, 2000-162899 and 2000-155476 disclose methods of fixing a contact pressure of a first transferer to an image bearer in a specific range so as not to apply a pressure more than predetermined to a toner image.
However, the most suitable pressure depends on an adherence amount of atoner, e.g., monochrome or multicolor-layered images. When the pressure is less or more than the most suitable pressure, the defective transfers cannot be sufficiently improved or even worsen.
In order to solve this problem, Japanese published unexamined applications Nos. 2002-014515 and 2005-024936 disclose methods of lowering the contact pressure to the downstream transfer site than that to the upstream transfer site, or differentiating the contact pressures to a black transfer site and the most upstream transfer site. Japanese published unexamined application No. 2006-301673 discloses a method of contacting a transfer unit including an intermediate transferer to an image bearer under its own weight to stably contact the transfer unit thereto.
However, controlling the pressure of a first transferer to an image bearer influences not only upon defective transfers but also on performance of a second transfer. When a sheet is a paper having low smoothness, the transferability of a toner differs due to concavities and convexities on the surface of the paper, resulting in scabrous or grainy images. Particularly, in an image forming apparatus using a two-component developer including a soft toner, the pressure of the first transferer needs considerate adjustment in consideration of the influence upon the second transfer.
Because of these reasons, a need exists for an image forming apparatus smoothly transferring toner images even from plural image bearers, using a two-component developer including comparatively a soft toner and a carrier.